According to the characteristics of RIS-aided URA, we propose a joint active device separation and channel estimation frame structure, as shown in Fig. REF . Specifically, the BS first estimates the RIS-BS channel and then recovers the transmitted messages of the active devices and estimates the corresponding device-RIS channels. Finally, the BS conducts downlink data transmission based on the estimated CSI.
Since the BS and the RIS are placed at fixed positions,
the BS-RIS channel \(\mathbf {U}\)  is quasi-static [1]}. As such, we only need to
estimate \(\mathbf {U}\)  once over a long period of time. The quasi-static RIS-BS channel can be efficiently estimated by the dual-link pilot transmission scheme proposed in [2]}, where the BS transmits pilots to the RIS via the downlink channel with a single-antenna. Then, the RIS reflects the transmitted pilots back to the BS via the uplink channel with a set of predesigned reflection coefficients.
Based on the received pilots, the BS can accurately obtain the estimate of the quasi-static RIS-BS channel, i.e., \(\hat{\mathbf {U}}\) .
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